The present invention relates generally to the reduction of multipath distortion in an automotive radio system, and more specifically to employing various multipath correction actions in accordance with measured vehicle speed.
The problem of multipath distortion in radio receivers is well known. Multipath occurs when radio frequency (RF) signals following direct and indirect (i.e., reflected) paths from a transmitter to a receiver interfere with each other at the receiver. Reflections can be caused by hills and buildings, for example.
Constructive and destructive interference of signals caused by interaction between the reflections and the direct line of sight transmission causes both signal distortion and rapid fluctuations in the received field intensity, especially in moving vehicles. Multipath is a particularly annoying problem in reception of FM stereo broadcasts because the stereo information is contained at higher frequencies more susceptible to multipath distortion.
Various prior art techniques are known for detecting and minimizing the effects of multipath interference. The occurrence of multipath is most often detected by monitoring the received signal strength for fluctuations and/or measuring high frequency noise in a demodulated output of the receiver. Various corrective actions are also known, such as reducing stereo separation by filtering high frequencies from the demodulated output or reducing the stereo blend control.
Diversity antenna systems are also employed to alleviate multipath effects. Since multipath effects are space,dependent, diversity receivers employ at least two antennas separated in space by an amount sufficient to receive the broadcast radio signals over independent signal paths, so that the separate antennas are not subject to the same multipath effects at the same time.
In spite of the application of these techniques to attack distortion caused by multipath interference, radio receiver performance continues to suffer from multipath.